Silicone rubber compositions which can be cured with the aid of an organic peroxide catalyst are known in the art. Such a curable composition, which is also called a rubber "base" by those skilled in this art, generally comprises a high viscosity silicone gum (which preferably has some vinyl functionality in its molecule), a reinforcing filler such as fumed silica and a "plasticizer."
A reinforcing filler is needed to bolster the mechanical properties of the inherently weak gum. High filler content (e.g., 10 to 75 parts by weight per 100 parts of the gum) is often required to achieve satisfactory mechanical properties in the final rubber, but there is an untoward consequence of its addition: the rubber base tends to become hard and brittle after even relatively short storage at ambient conditions. This phenomenon, known variously in the art as crepe hardening, crepe aging or structure formation, makes further processing of the rubber base (e.g., compounding, molding, extrusion) impractical or, at best, difficult. Crepe hardening must therefore be kept to a minimum. In order to attain this end, a plasticizer such as a hydroxy-terminated short chain siloxane is typically incorporated in the formulation and its addition allows the rubber base to maintain a relatively fluid condition for subsequent processing. An organic peroxide is then thoroughly dispersed in the rubber base to provide a rubber "stock" which, in turn, can be fabricated into a desired shape and cured at elevated temperature to a finished silicone rubber part. Although the above described silicone rubber compositions exhibit relatively high tensile strength, the amount of reinforcing filler needed for this purpose also tends to impart a high modulus to the cured rubber. This is undesirable in many applications, such as gaskets, O-rings, connectors, wire and cable, as well as various extruded and molded parts where flexibility is a key requirement. Moreover, even with high filler loadings, cured silicone rubbers are notorious for their poor tear strength relative to cured organic rubbers. These limitations further restrict the arena of application for the silicone rubbers and represent unmet needs in the commercial exploitation thereof.